As is known, the fraction of hemoglobin in arterial blood which is in the form of oxygenated Hb and which is often referred to as the oxygen saturation of blood SaO2, can be measured by a photo-electric measuring device known as oximeter. As will be explained later on in more detail, the saturation of oxyhemoglobin (hemoglobin combined with oxygen) in arterial blood in the tissue of a living body can be detected by means of light of at least two different wavelengths passing through or reflecting from the tissue so as to be modulated by the pulsatile blood flow therein. The reason for the variation of the transmission or reflection of the light through the blood in that the absorption coefficient of oxyhemoglobin is different from that of deoxygenated hemoglobin for different wavelengths of light. Thus, the measurement of the amount of light passing through or reflected from a member of the body having a pulsatile blood flow therein can be used for indicating the saturation of hemoglobin in the blood. As is also known in the art, fixed absorbers of the tissue reduce the amount of light passing through or being reflected from the body by an essentially constant amount which can be regarded as a DC component. A pulsatile component or AC component of the light at different wavelengths passing through or reflecting from the body is primarily caused by the effect the changing arterial blood volume within the member of the body has on the passing or reflected light.
Consequently, most of the prior art oximeters eliminate the DC component from the signals analysed and only utilize the pulsatile component for the calculation of the oxyhemoglobin saturation. For example, U.S. Pat. No. 4,167,331 (Nielsen) discloses a technique of determining the pulse rate and arterial oxygen saturation by means of a three-wavelengths absorbence oximetry, where the DC components of the respective signals are suppressed or eliminated by bandpass filters. The bandpass filtering of the signal results in a signal distortion negatively affecting the accuracy in determination of the pulse rate and the oxygen saturation. This prior art technique is also involved with another problem resulting from the fact that this technique starts from the assumption that the AC component is exclusively caused by a change in the volume of the blood. However, this assumption underlying the prior art oximeter as exemplified by U.S. Pat. No. 4,167,331 appears to be too rough as the following factors may influence the oximetry signals: venouse pulsation caused by an excessive contact pressure force of the sensor, the influences of electrical noises caused by radio frequency apparatus for the surgery, optical interferences generated by adjacent light sources of pulsatile character, misplacements of the sensor itself at the measuring position as well as artifacts due to the patient's motions.
Although most of the prior art oximeters are equipped with averaging circuits for averaging measured values concerning the oxygen saturation and the pulse frequency over a period of seconds, or at least a plurality of pulses before generating a display of these values, noisy peaks of the above-indicated types nevertheless have a certain negative influence on the accuracy which can be achieved with this prior art oximeter.
For enhancing the accuracy by particularly processing the signals used in oximetry, EP-262,778-A1 (Physio-Control Corporation) discloses a method of processing oximetry signals by firstly searching for a sustained positive sloping region of the signal, determining the respective times before and after the occurence of a slope reversal, searching for a maximum which is identified as a positive peak after a first positive slope and searching for a negative peak after occurrence of a negative sloping region. For improving the accuracy, signals failing to comply with predetermined requirements are rejected. For example, pulse amplitudes not satisfying a predetermined amplitude selection criterion are rejected. The rejection criterion may also include a systolic interval template defining an allowable systolic interval range for defining a time interval between positive and negative peaks. Although this prior art oximetry method appears to be suitable for enhancing the measuring accuracy by eliminating or suppressing the influence of certain noisy signals, it is not suitable for eliminating an effect on the measurement accuracy by motional artifacts or dicrotic pulses.